BMI Calculator
Calculate your Body Mass Index (BMI) to check whether you are at a healthy weight for your height.
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What Is a BMI Calculator?
A BMI calculator divides your weight in kilograms by the square of your height in metres. The result is a single number that places you in one of four NHS weight categories: underweight, healthy weight, overweight, or obese.
The NHS and the World Health Organisation use BMI as a first-line screening tool for weight-related health risks. A BMI between 18.5 and 24.9 indicates a healthy weight for most adults. Values above or below that range signal higher risk of conditions such as type 2 diabetes, heart disease, and joint problems.
Enter your height and weight below to get your BMI in seconds. Compare the result against the NHS category table further down the page.
How Do You Use This BMI Calculator?
Enter your height in centimetres and weight in kilograms. Click Calculate to see your BMI value and the weight category it falls into.
- Enter your height in centimetres in the height field.
- Enter your weight in kilograms in the weight field.
- Click the Calculate button to generate your result.
- Read your BMI value in the output panel.
- Check the weight category displayed beside the number.
- Compare your result against the NHS ranges in the table below.
How Does the BMI Calculator Formula Work?
The formula used: BMI = weight (kg) / height (m)²
The BMI formula divides weight by height squared: BMI = weight (kg) / height (m)².
Convert height from centimetres to metres first. For 175 cm, divide by 100 to get 1.75 m. Square that value: 1.75 × 1.75 = 3.0625. Divide weight by the squared height: 80 / 3.0625 = 26.1.
The result is unitless. The NHS maps it to four categories: under 18.5 (underweight), 18.5–24.9 (healthy), 25–29.9 (overweight), and 30+ (obese).
What Are Some Example Calculations?
For a person 175 cm tall weighing 80 kg: BMI = 80 / (1.75)² = 26.1, which falls in the Overweight category.
Healthy-weight adult
Height 170 cm, weight 65 kg. Convert height: 1.70 m. Square: 2.89. BMI = 65 / 2.89 = 22.5.
BMI 22.5 — Healthy weight category.
Overweight adult
Height 180 cm, weight 90 kg. Convert height: 1.80 m. Square: 3.24. BMI = 90 / 3.24 = 27.8.
BMI 27.8 — Overweight category.
Underweight adult
Height 165 cm, weight 48 kg. Convert height: 1.65 m. Square: 2.7225. BMI = 48 / 2.7225 = 17.6.
BMI 17.6 — Underweight category.
When Should You Use a BMI Calculator?
Use this BMI calculator before a GP appointment to get a baseline weight-status reading. GPs and practice nurses routinely record BMI during health checks. Having your number ready speeds up the consultation.
Check your BMI after a sustained change in diet or exercise to track progress over weeks or months. Avoid daily checks — weight fluctuates by up to 2 kg in a single day due to water retention and food intake.
What Do These Terms Mean?
How Do the Options Compare?
| BMI Range | Category | NHS Weight Status | Health Risk Level |
|---|---|---|---|
| Below 18.5 | Underweight | Below healthy range | Increased (nutritional deficiency) |
| 18.5 – 24.9 | Healthy weight | Within healthy range | Low |
| 25.0 – 29.9 | Overweight | Above healthy range | Increased |
| 30.0 – 34.9 | Obese class I | Obese | High |
| 35.0 – 39.9 | Obese class II | Obese | Very high |
| 40.0 and above | Obese class III | Severely obese | Extremely high |
What Are the Best Tips to Know?
- Weigh yourself first thing in the morning before eating for the most consistent reading.
- Measure height without shoes and stand flat against a wall.
- Use the same scales each time to reduce measurement variation.
- Record results monthly rather than daily to spot genuine trends.
- Pair BMI with a waist circumference measurement for a fuller picture of health risk.
What Mistakes Should You Avoid?
- Entering height in metres instead of centimetres, producing an impossibly high BMI.
- Forgetting to remove shoes before measuring height, which inflates the reading by 2–3 cm.
- Treating BMI as a diagnostic tool rather than a screening indicator.
- Ignoring that BMI overestimates body fat in muscular individuals and underestimates it in older adults.
Frequently Asked Questions
What is a healthy BMI range?
A healthy BMI is between 18.5 and 24.9. Below 18.5 is considered underweight, 25 to 29.9 is overweight, and 30 or above is classified as obese according to NHS guidelines.
Is BMI accurate for everyone?
BMI does not account for muscle mass, age, sex, or ethnicity. Athletes may have a high BMI due to muscle. It is a screening tool, not a diagnostic measure.
How often should I check my BMI?
Checking your BMI every few months is sufficient for most people. Focus on overall health habits rather than frequent BMI monitoring.
Does BMI differ for children and teenagers?
Yes. Child BMI uses age- and sex-specific percentile charts rather than fixed adult thresholds. A child in the 85th to 94th percentile is classified as overweight. Use an age-adjusted calculator for anyone under 18.
What BMI thresholds apply to South Asian and Black adults?
The NHS recommends lower thresholds for South Asian, Chinese, and Black African/Caribbean adults. A BMI of 23 or above indicates increased risk, and 27.5 or above indicates high risk, compared with 25 and 30 for the general population.
Can BMI tell me my body fat percentage?
No. BMI estimates weight status but does not measure body fat directly. DEXA scans, skinfold callipers, and bioelectrical impedance devices provide body fat percentage readings.
Why do doctors still use BMI if it has limitations?
BMI is fast, free, and requires only a scale and a tape measure. Population studies show strong correlation between high BMI and increased disease risk. Doctors pair it with waist circumference and blood tests for a fuller assessment.
What should I do if my BMI is outside the healthy range?
Book a GP appointment to discuss the result. A GP can assess additional risk factors such as blood pressure, cholesterol, and family history before recommending dietary or activity changes.
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